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Register a new userRashba spin-splitting of single electrons and Cooper pairs
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Electric weak links, the term used for those parts of an electrical circuit that provide most of the resistance against the flow of an electrical current, are important elements of many nanodevices. Quantum dots, nanowires and nano-constrictions that bridge two bulk conductors (or superconductors) are examples of such weak links. Here we consider nanostructures where the electronic spin-orbit interaction is strong in the weak link but is unimportant in the bulk conductors, and explore theoretically the role of the spin-orbit active weak link (which we call a Rashba spin splitter) as a source of new spin-based functionality in both normal and superconducting devices. Some recently predicted phenomena, including mechanically-controlled spin- and charge currents as well as the effect of spin polarization of superconducting Cooper pairs, are reviewed.
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We investigate theoretically the properties of a weak link between two superconducting leads, which has the form of a non-superconducting nanowire with a strong Rashba spin-orbit coupling caused by an electric field. In the Coulomb blockade regime of single-electron tunneling, we find that such a weak link acts as a spin splitter of the spin states of Cooper pairs tunneling through the link, to an extent that depends on the direction of the electric field. We show that the Josephson current is sensitive to interference between the resulting two transmission channels, one where the spins of both members of a Cooper pair are preserved and one where they are both flipped. As a result, the current is a periodic function of the strength of the spin-orbit interaction and of the bending angle of the nanowire (when mechanically bent); an identical effect appears due to strain-induced spin-orbit coupling. In contrast, no spin-orbit induced interference effect can influence the current through a single weak link connecting two normal metals.
We measure an aluminum superconducting double quantum dot and find that its electrical impedance, specifically its quantum capacitance, depends on whether or not it contains a single broken Cooper pair. In this way we are able to observe, in real time, the thermally activated breaking and recombination of Cooper pairs. Furthermore, we apply external microwave light and break single Cooper pairs by the absorption of single microwave photons.
We have performed density functional theory calculation and tight binging analysis in order to investigate the mechanism for the giant Rashba-type spin splitting (RSS) observed in Bi/Ag(111). We find that local orbital angular momentum induces momentum and spin dependent charge distribution which results in spin-dependent hopping. We show that the spin-dependent interatomic-hopping in Bi/Ag(111) works as a strong effective field and induces the giant RSS, indicating that the giant RSS is driven by hopping, not by a uniform electric field. The effective field from the hopping energy difference amounts to be ~18 V/{AA}. This new perspective on the RSS gives us a hint for the giant RSS mechanism in general and should provide a strategy for designing new RSS materials by controlling spin-dependence of hopping energy between the neighboring atomic layers.
We suggest a way to characterize the coherence of the split Cooper pairs emitted by a double-quantum-dot based Cooper pair splitter (CPS), by studying the radiative response of such a CPS inside a microwave cavity. The coherence of the split pairs manifests in a strongly nonmonotonic variation of the emitted radiation as a function of the parameters controlling the coupling of the CPS to the cavity. The idea to probe the coherence of the electronic states using the tools of Cavity Quantum Electrodynamics could be generalized to many other nanoscale circuits.
In inversion-asymmetric semiconductors, spin-orbit coupling induces a k-dependent spin splitting of valence and conduction bands, which is a well-known cause for spin decoherence in bulk and heterostructures. Manipulating nonequilibrium spin coherence in device applications thus requires understanding how valence and conduction band spin splitting affects carrier spin dynamics. This paper studies the relevance of this decoherence mechanism for collective intersubband spin-density excitations (SDEs) in quantum wells. A density-functional formalism for the linear spin-density matrix response is presented that describes SDEs in the conduction band of quantum wells with subbands that may be non-parabolic and spin-split due to bulk or structural inversion asymmetry (Rashba effect). As an example, we consider a 40 nm GaAs/AlGaAs quantum well, including Rashba spin splitting of the conduction subbands. We find a coupling and wavevector-dependent splitting of the longitudinal and transverse SDEs. However, decoherence of the SDEs is not determined by subband spin splitting, due to collective effects arising from dynamical exchange and correlation.
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